1. Field of the Invention
This invention relates to the spontaneous and self-sustained reduction and accumulation of precious metals, such as gold (AU), platinum (Pt), and palladium (Pd) from acid solutions by electroactive polymers, such as polyaniline (PAN), polypyrrole (PPY) and their analogs and derivatives, which are capable of assuming a number of intrinsic redox states. A typical polymer film is capable of accumulating more than 5 times its own weight of a precious metal.
2. Description of the Related Art
Gold is a precious metal which has always been in high demand for its decorative and ornamental purposes for centuries. Together with other precious metals, such as platinum and palladium, these metals are of strategic importance for the present day high-technology and electronic industries. Thus, recovery of these precious metals from primary and secondary sources, such as natural ores, leach solutions, electronic scraps, and waste electroplating solutions, has become an important technology. Recovery of these and other precious metals by less energy intensive processes, such as gold reduction or accumulation by polymeric adsorbents, by biomass and biomaterials, by activated carbon, and by electroless plating are well-documented in the literature. The have been described, for example, in "Advances in precious Metals Recovery", edited by N. Arbiter and K. N. Han, Gordon and Breach Science Publishers, N.Y. (1990); and in G. J. McDougall and R. D. Hancock, Minerals Sci. Engng, 12, 85 (1980). Cyanidation, until recently, has been the only alternative to remove small particles of gold from slime on an industrial scale. Gold chlorination and gold extraction from acid solutions have risen to prominence during the last two decades, as this recovery route does not have the adverse environmental effects of cyanidation. Thus, as the demand for gold and other precious metals increases, extraction of these materials from their acid solutions must be accomplished with greater efficiency.
In a parallel development, electroactive (conductive) polymers have emerged in the last two decades as a new class of materials with interesting electrical and electronic properties. A number of electrically conductive or semiconductive polymeric materials are known. They have been described, for example, in "A Handbook of Conducting Polymers" Vols I and II, edited by T. Skotheim, Marcel Dekker, N.Y. (1986). Polymers with conjugated backbones are of particular interest. The electrical conductivities of such materials may be made to undergo a metal-like transition, via the process of chemical or electrochemical doping (oxidation, reduction and protonation, etc.). Four main classes of such conjugated polymers have been identified, viz., poly(acetylene) and its derivatives, poly(phenylene) and its derivatives, poly(heterocyclic) polymers, and the aniline polymers. Of these polymers, polyaniline (PAN) and polypyrrole (PPY) and their derivatives have been of particular interest because of their high electrical conductivity, environmental stability and interesting intrinsic redox properties associated with the chain nitrogens. The last properties have been studied in detail in E. T. Kang, K. G. Neoh and K. L. Tan, "The Intrinsic Redox States in Polypyrrole and Polyaniline: a Comparative Study of XPS", Surf. Interf. Anal. 19, 33 (1992). Thus, by coupling the increase in the intrinsic oxidation state of these conductive polymers and their spontaneous protonation and reduction in acid solution with the decrease in the oxidation state of the metal, the process is capable of self-sustained electroless precipitation of precious metals in elemental form from acid solution.
Activated carbon was first used as a precipitant for gold in the chlorination process as early as 1880, and for the recovery of gold from cyanide solution in the 1920's. Maximum Au uptake was about 500 mg/g C. The processes have been reviewed by G. J. McDougall and R. D. Hancock, Minerals Sci. Engng, 12, 85 (1980).
The phenomena of sorption and biosorption have been utilized in the extraction of metallic species from solutions. Living and non-living cells and polymers have been used to concentrate metallic anions from their aqueous environment. The common phenomenon involves rapid and reversible physical/chemical adsorption of metals in the polymer and cellular structure, in combination with complexation, ion-exchange, and/or microprecipitation.
In U.S. Pat. No. 4,769,223, Sep. 6, 1988, a process is disclosed for removing Au ions from aqueous solution or suspension by biomass derived from the genus Sargassum (e.g. Sargassum natans), a brown sea-water algae. The maximum metal uptake is in the order of 420 mg per g dry weight of the biomass at pH 2.5. The adsorbed metal is eluted using thiourea and ferric ammonium sulfate as eluant.
In U.S. Pat. No. 4,289,531, Sep. 15, 1982, Pt, Pd and other precious metals are recovered from aqueous media containing the metal ions by contacting the solution with a proteinaceous material selected from the group consisting of feathers, hair, hoof meal and horn meal. Maximum recoveries of precious metals occur within the preferred pH range of 2 to 3. Typical recovery efficiencies for Pt from 100 ppm chloroplatinic acid are about 70-90 mg Pt/g of contact materials.
In Canadian Patent CA 2,030,900, May 28, 1991, cyanide-generating microorganisms, such as algae, bacteria or fungus, are used to leach powder Au ores for low-cost Au recovery and reconcentration. Au recovery is 76-96 ppm vs. 0.77 ppm in the absence of culture.
In Japan Kokai Tokyo Koho JP 02080528, Mar. 20, 1990, HCl solutions containing Ag and Zr ions are contacted with a cationic resin for sorption of the Ag+ ion.
In Japan Kokai Tokyo Koho JP 02015128, Jan. 18, 1990, aqueous gel containing persimmon tannin and aldehyde or acid is used for the recovery of noble metals. Thus, aqueous dilute HAuCl.sub.4 solution (pH 3) containing 10 ppm Au was contacted for 1 hour with the gel to recover 98.1% of Au.
In U.S.S.R. Patent SU 1,556,735, Apr. 15, 1990, the process involves contacting the solution at various temperatures with S-containing polymeric sorbent, such as a sulfonated phenolic polymer, to increase the efficiency of Ag recovery.
In Danish Patent DK 156075 B, Jun. 19, 1989, Au is extracted from aqueous solutions containing Au, Cu and Fe in a thiourea complex from ore extraction by sorption with an acidic cation exchanger (sulfonated polystyrene resin), elution and then reductions. Overall Au recovery efficiency is at 90% for a solution containing 0.5 mg/l thiourea, 0.5 mg/l H.sub.2 SO.sub.4, 0.5% Fe.sub.2 (SO.sub.4).sub.3, 160 mg/l Cu and 70 mg/l Au.
In Australian Patent Specification, AU 564754, Aug. 27, 1987, precious metals (especially Au) in oxides or carbonaceous ores are leached with aqueous cyanide solution and recovered by adsorption on activated C. The loaded C pulp is desorbed in dilute NaOH solution at elevated temperature and pressure. The hot solution is cooled for precipitation of the precious metal and conventional recovery.
In East German Patent DD 238033, Jun. 4, 1985, Pd is selectively separated from acid solutions, especially waste solutions from reprocessing of nuclear fuels, by precipitation with ferrocyanides or sorption on ferrocyanides.
In PCT International Application WO 8603480, Jun. 19, 1986, microorganisms, such as algae on glass wool or SiO.sub.2 gel, are applied under controlled conditions of pH and salt concentration to selectively recover Au, Ag, Pt or Hg. The binding of Au in 10.sup.-4 M AuCl.sub.4.sup.- solution is high for algae concentration&gt;1 mg/ml, and is insensitive to pH at 2-9.5. The bound Au.sup.3+ or Au.sup.+ is eluted as Au.sup.+ by thiourea at 10.sup.-4 M in 0.01M HCl. The Au recovery for trace Au.sup.+3 in 0.01 HCl feed solution is 75-100% when passed through a column loaded with algae supported on polyacrylamide.
In Czech Patent CS 220145, Oct. 15, 1985, Au and Pt-group metals are recovered from solutions by sorption on polymer gel containing a thiirane group or groups formed by their decomposition by NH.sub.3. Thus, a macroporous 2,3-epithiopropyl acrylate-ethylenedimethacrylate copolymer containing 49% solid and having surface area of 76 m.sup.2 /g absorbs Au in 24th from a medium containing 2.1M HCl at 100 mg Au/g dried weight without trapping of Cu or other metals.
In German Offen. DE 3401961, Aug. 23, 1984, a cyanide-free hydrometallurgical method is proposed for the recovery of Ag and Au from ores and other raw materials by leaching with thiourea in an acid medium. The precious metals are selectively adsorbed on activated C or cation exchanger, eluted with thiourea or acids, and recovered by electrolysis.
In East German Patent DD 200792, Jun. 15, 1983, Pd is recovered from HNO.sub.3- containing nuclear fuel regeneration solution by sorption with a chelate-forming ion exchange. For Pd loading of 45 mg/0.5 g of ion exchanger, the recovery of Pd in the desorption stage is 96%.
In U.S. Pat. No. 3,736,126, May 29, 1973, AuCl.sub.4.sup.- anions are separated from other metals in strong acid solution as they are retained in an adsorption bed of a polymer of lower aliphatic esters of acrylic or methacrylic acid. The absorbed Au can be stripped with dimethylformamide or a mixture of 1M HCl with 2.5 times its volume of Me.sub.2 CO.
In U.S.S.R. Patent Su 1,956,29, Sep. 14, 1970, Pt is extracted from acid solution by sorption. To increase the effectiveness of the desorption of Pt, an amphoteric resin containing amino acid groups is used as the sorbent.